Microwave transmission control tubes and methods



Jan. 7, 1958 I P. E. GATES 2, 1

MICROWAVE TRANSMISSION CONTROL 'ruis'Es AND METHODS Filed 00 1, 1952 $2 EN? .56, 7 xi I ATTORN Y United States Patent MICROWAVE TRANSMISSION CONTROL TUBES AND METHODS Paul Gates,rDanvers, Mass., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application October 1, 1952, Serial No. 312,604

7 Claims. (Cl. 313-198) The present invention relates to gaseous electric-discharge devices; in particular, the present invention contjempl'ates the provision of improved attenuator and switching tubes for microwave circuits, and to amethod of making such tubes.

A wide Variety of gaseous discharge devices, for example, attenuators, transmit-receive tubes, and anti-transmitrecei've tubes depend for their performance on a sustained glow discharge in a gaseous fill at a pair of discharge electrodes constituting a break-down discharge gap. While the, more general scope of application of the invention will be understood, the'illustrative disclosure that follows is directed to the transmit-receive tube.

Intransmit-receive tubes, commonly designated TR tubes, the break-down discharge or gap electrodes are arranged across a microwave transmission path, usually defined by a waveguide. When a signal of a high energy level is transmitted via the waveguide, a correspondingly high oscillating voltage is impressed on the discharge electrodes which initiates an intense ionized discharge in the gaseous fill and controls the transmission of the micro- .wave signal. The ionization produces a breakdown effective as a protective barrier or to control the paths and attenuation of transmission.

In order to render transmit-receive tubes or duplexers more effective, as well as to control attenuators it is advanta'geous to maintain a limited volume of the gaseous fill in the region of the discharge gap in a weakly ionized state. The weak ionization, that is, the glow discharge, is sustained during low level signal conditions and facilitatesthe intense ionized discharge or breakdown across thetransmi'ssionpath when high level microwave energy is impressed on the waveguide. For the purpose of obtaining the glow discharge an additional electrode termed the keep-alive or keep-alive electrode, is. arranged in close proximity to the discharge gap electrodes, either concentrically to and within one of the discharge electrodes, or externally of the discharge electrodes adjacent to the gap. By applying a suitable potential between the keep-alive and the appropriate discharge electrode, weak ionization of a limited volume of the gas fill is established in a restricted region.

In addition to rapid breakdown in dependence on the presence of a high energy signal or burst, it is desirable that microwave devices of the aforesaid character have a relatively quick recovery time. Heretofore, this has been accomplished by using as a gaseous fill a mixture of an inert gas, preferably a noble gas such as argonwhose ions combine'only with each other after ionization, together with a quenching gas, such as water. In operation, when exposed to such gaseous atmosphere and the ionizing potential, the keep-alive electrodes commonly used heretofore tend to deteriorate and limit the useful life of the tubeto several hundred hours, even at the relative low currents, i. e. 10 to 50 microamperes. Tube life is also limited by clean-up of the water-vapor used as a quenching gas in many tubes of this class. When it is necessary "ice to increase the glow discharge, as in attenuator tubes and in broadband transmit-receive tubes having resonant cavities of low Q design, the risk of progressively destroying the keep-alive electrode and cleaning up the water vapor is greatly increased, since currents of the order of 1000 microamperes are needed. Accordingly, resort to keep alive materials capable of withstanding relatively high current densities, especially when exposed to a watercontaining gaseous fill, have been suggested.

Among the materials considered are refractory metals such as molybdenum, and tungsten. With these materials the desired currents have not attained over a reasonably long tube life. Size limitations imposed on the keep-alive due to the physical geometry of the conventional electrode constructions renders inexpedient any great increase in the size of the keep-alive for reducing current density. Other materials that showed promise of resisting deterioration of the electrode and gas fill have not been considered adaptable to use as a keep-alive. Titanium oxide, a fragile ceramic, is one such material that has been rejected because of fabrication difficulties.

Accordingly, it is an object of the present invention to provide novel microwave gaseous discharge devices of improved operating characteristics. More particularly, the present invention aims at the provision of microwave gaseous discharge devices having a novel electrode effective as a keep-alive yet having properties promoting long tube life.

A still further object of the present invention resides in the provision of a microwave device, incorporating a keep-alive electrode suitable for maintaining a high level of glow discharge, constructed to minimize deterioration of the keep-alive electrode, and of an aqueous quenching mixture where used.

Yet a still further object of the present invention is the provision of a novel process for constructing a keep-alive electrode. In particular, a feature of the present invention resides in the provision of a novel method of incorporating fragile materials such as titania having desirable keepalive properties in a practical electrode construction.

The above objects and further features and advantages of the present invention will become apparent from the following detailed description of an illustrative embodiment and process shown in the accompanying drawings, wherein:

Fig. 1 is a flow diagram showing the processing of an insert forming a part of a novel keep-alive electrode;

Fig. 2 is a further flow diagram showing the assembly of the illustrative keep-alive electrode; and

Fig. 3 is a longitudinal cross-section of a transmitreceive tube containing a keep-alive electrode constructed in accordance with the process of Figures 1 and 2.

Referring now to the drawings and in particular to Fig. 3, there is shown a rectangular length of Waveguide 10, having flanges 12, 12' for connection to a microwave system. The ends of the waveguide 10 are provided with resonant windows 14, 14 each of which includes a center glass or ceramic body 16, and a metal bezel or frame 18. One or more opposite pairs of conical discharge electrodes 20, 22, afford a series of discharge gaps at spaced intervals along the waveguide 10, and the gap electrodes have metallic interconnection via the waveguide wall. Electrode 22 is of closed deformable construction for critical adjustment by engagement from the exterior of the waveguide 10 through the complementary body 24 fixed in electrode 22. Associated with discharge gap 26, defined by the opposite electrodes 2t) and 22, is a laterally disposed pair of plates (not shown) of conventional construction which provide an iris having a center opening for the gap electrodes. The iris and the gap electrodes constitute a resonant aperture through which microwave energy of low level can be transmitted from one window to the other. The respective discharge gaps and irises are separated from each other along the Waveguide by a spacing commonly being a distance equal to an effective quarter wave length at the center frequency of the band for which the device is designed.

Electrodes are hollow in the preferred construction and each contains a keep-alive electrode 30 having an inner end 30' limited to a spacing of a few thousandths of an inch from the opening 20' in discharge electrode 20, both radially and end-wise. Discharge electrode 20 is accordingly closer to the other discharge electrode 22 than keep-alive electrode 30 is to discharge electrode 22 and only the relatively small inner end 30 of the keepalive electrode 30 is exposed to the gaseous fill of the sealed waveguide 10 in the region of the discharge gap 26. The waveguide 10 is conventionally filled with an electron capture gas preferably a noble gas such as argon, mixed with a quenching gas such as water at a total pressure of the order of 25 millimeters of mercury.

In operation, a current is passed between the keep-alive electrode and the discharge electrode 20. In such transmit-receive tubes, it is normal to apply several hundred volts between the keep-alive 30 and the gap electrodes. When the required potential is applied between the keep-alive and the waveguide structure a small volume of ionized gas is locally established in a confined region adjacent gap 26 which is somewhat effective to attentuate low level signals transmitted along the waveguide path, the attenuation being a function of the degree of ionization. Upon incidence of high level signal bursts the liimted volume of ionized gas promotes an abrupt formation of intensely ionized discharges thereby to switch the Waveguide transmission path from one in which low level signals are transmitted to one in which high level signals are almost entirely reflected. The use of such devices in radar systems and in single station receiver-transmitter units is well understood by those well skilled in the art, and requires no elaboration here.

When it is desired to increase the intensity of glowdischarge as in attenuator tubes, it is usual to greatly increase the ionizing keep-alive current. As this current increases there will be a proportionate increase in current density which might normally result in the accelerated deterioration of the keep-alive. This is of special concern in the presence of a gas fill containing water vapor where the water vapor is cleaned up or removed from the mixture during life of the tube. Accordingly, it is desirable to employ a keep-alive electrode having an increased current carrying capacity, and one that does not enter into the clean-up action.

In accordance with features of the present invention the keep-alive electrode 30 (Figs. 2 and 3) is formed with a rod 32, including a tapered or conical portion 32 at one end which is provided with an axial bore 34 forrning the tapered portion 32' into a shell 36. Within the bore 34 there is an insert or tip 40 of a semiconducting ceramic, specifically, partially reduced titanium mono or dioxide. The insert 40 has a metal plating on its cylindrical surface at 42, the end face 40' thereof being exposed. A glass sheath or insulating head 38 is sealed continuously on the outer surface of the shell 36 and along a portion of the rod 32 that extends beyond shell 36. The sheath 38 serves to prevent a discharge between the keep-alive electrode 32 and the outer metallic structure of the hollow gap electrode 20 except in the immediate region of opening 20'. This keep-alive electrode 30 is hermetically sealed by glass button 44 to sleeve or collar 46 that is brazed hermetically to waveguide 10 and rod 32 is fixed concentrically of cone 20 and critically fixed in endwise spacing relative thereto. The sleeve 46 is of a suitable glass-to-metal sealing alloy, such as the well known iron-nickel-cobalt alloys.

The detailed procedure and assembly of the keep-alive and gap electrode are more fully described and claimed 4 in my copending application Serial No. 237,258, filed July 17, 1951, now Patent No. 2,740,186. Reference is also made to copending application Serial No. 226,483, filed May 15, 1951, by Richard A. Hagan, now Patent No. 2,768,320. An important feature of the present invention relates to novel keep-alives and to their manufacture.

Titania as a ceramic is an insulator and is quite brittle. It can be rendered sufficiently conductive for carrying the order of current required in switching and attenuator tubes, by firing or baking at high temperature for a short period. In order to utilize such material as a keepalive I have formed the fired titania as an insert. It is secured in place by first applying a preliminary plating of metal followed by a brazing operation. This assembly of the insert to its supporting lead is excellent both mechanically and electrically. The plating operation is feasible after the titania has been fired, because, even with its relatively poor conductivity the titania can be plated with a sufficient amount of metal for the subsequent brazing operation. In this way, the physical properties of the titania are effectively made available for improving the life of the tube without risking mechanical failure, and a metal support in which it is received is selected for sealing through glass as a terminal.

The keep-alive electrode may be assembled in the following exemplary manner. Initially, and as seen in Fig. 1, an extruded rod of titanium oxide a is fired in hydrogen at approximately 1150 degrees centigrade for about 3 minutes. The fired rod 11 is prepared for brazing by the application thereto of metal plating in accordance with techniques well understood, per se. For example, superposed layers of nickel and copper may be applied by electro-plating. The plated rod is then divided into inserts 40 of a prescribed length having plating 42 on the cylindrical surfaces thereof. These inserts may be only 0.016 inch in diameter by inch long.

Referring now to Fig. 2, there is shown the bored rod 32 of the keep-alive which is suitably cleaned of oxides as, for example, by firing in a furnace having a hydrogen atmosphere. Rod 32 receives the insert 40 with the end face 40 exposed, and with part of its length projecting beyond the end of rod .32. The plating acts as a brazing metal, and is melted in an inert or a reducing atmosphere by radio frequency induction to unite the insert 40 to the rod 32. The plating on the lateral surfaces of the projecting insert melts away from the end 40', so that only the titania is later exposed in the discharge region. Rod 32 with the insert brazed therein is provided with the insulating sheath 38, suitably of glass, on all of the exposed surfaces thereof and on the lateral portion of insert 40 except for the end face 40'. Any glass covering the end 40 is ground away. The completion of the assembly follows according to my above mentioned copending application.

Such keep-alive has all the advantage of the insert material as to reducing deterioration of the electrode and its fill, and all the advantage of the strong, electrically conductive glass-sealing alloy leads in tubes having allmetal keep-alives. A construction and method has been disclosed that is highly effective to unite the materials rigidly into a tiny but highly effective, precisely proportioned glow-discharge electrode. Taking advantage of the principles disclosed, those skilled in the art may readily introduce detailed modification and may make varied application of the invention, and accordingly the appended claims should be broadly construed, as inconsistent with the spirit and scope of the invention.

What is claimed is:

1. The method of making a keep-alive electrode as sembly comprising the steps of firing a titania insert, plat ing the titania insert, mounting said plated insert at one end of a metallic rod with the face of said titania insert exposed, brazing said plated titania insert to said metallic rod, and heading said metallic rod with an insulating sheath to cover said rod and insert except in a confined region occupied by said exposed face of said insert.

'2. The method of making a keep-alive electrode assembly comprising the steps of forming a bore in a metallic rod of a glass-sealing metal, plating an insert of a semiconducting ceramic with a material capable of being mechanically united to said metallic rod, placing said i11- sert in said bore in an adjusted position relative to said metallic rod with an end face of said insert exposed brazing said insert to said metallic rod in said adjusted position, and beading said metallic rod with a glass sheath to insulate said metallic rod except in a confined region substantially occupied by said end face of said insert.

3. The method of making a keep-alive electrode comprising the steps of forming a rod with a metallic shell of a metal-to-glass sealing alloy, firing a titania insert and electro-plating the lateral surface of the insert, inserting and brazing the plated insert Within said shell With said end face exposed, and sheathing said rod with a glass layer except for said end face of said insert.

4. The method of making an electrode, including the steps of providing a metal support with a recess, forming and firing a titania insert and depositing a metal film on the lateral surface thereof, uniting the insert to the support with brazing heat and atmosphere and with a portion of the insert projecting from the support, the metal film on the projecting portion inherently receding from the end of the insert, and applying an insulating sheath to the support, the metal film and the lateral surface of the projecting insert.

-5. A keep-alive electrode including a supporting lead having a terminal end and a discharge end, a glow-discharge semiconducting ceramic cathode at said discharge end, bonding metal about said ceramic cathode and united to the discharge end of said lead, and a glass sheath completely covering the discharge end of said lead and said bonding metal, and further covering the ceramic cathode adjacent the bonding metal.

6. A keep-alive electrode including a supporting lead having a terminal end and a discharge end, a glow-discharge semiconducting titania cathode at said discharge end, a plated sleeve about said titania cathode and united to the discharge end of said lead, and a glass sheath completely covering the discharge end of said lead and said plated sleeve, and further covering the titania cathode adjacent the plated sleeve.

7. A keep-alive electrode including a supporting terminal rod having a terminal end and a discharge end, said terminal rod having an axial bore at its discharge end, a glow-discharge semiconducting ceramic cathode inserted in said bore and having a portion projecting therefrom including a discharge surface, an annular metal sleeve plated on said cathode and united to the surface of said bore, said sleeve extending slightly beyond the discharge end of said rod, and an insulating glass sheath covering the surfaces of the discharge end of said rod, the portion of said metal sleeve extending beyond the discharge end of the rod, and the projecting portion of said cathode except for said discharge surface.

References Cited in the file of this patent V UNITED STATES PATENTS 2,159,791 Fruth May 23, 1939 2,303,514 Toepfer Dec. 1, 1942 2,404,116 Wolowicz et a1 July 16, 1946 2,454,761 Barrow et al Nov. 30, 1948 2,456,563 McCarthy Dec. 14, 1948 2,582,202 Jacob Jan. 8, 1952 2,617,957 *Scott Nov. 11, 1952 2,631,255 Stavro Mar. 10, 1953 2,680,207 Booth June 1, 1954 

